The invention in this case relates to a new and improved magnet-to metallic body seal, to a magnetic device comprising such a seal and to an improved method of joining a permanent magnet to a metallic body.
It is known to bond permanent magnets to metallic bodies, including metal shafts, by epoxy adhesives. However, when the device utilizing such a magnetic device is a device such as an actuator is employed over a wide temperature range, for example at temperatures as low as -40.degree. C. and as high as 200.degree. C., under conditions involving high speed rotation with many changes in direction and speed of rotation, it has been that there are frequent failures in the permanent magnet-metal shaft seal.
The Cole et al, U.S. Pat. No. 3,793,705 provides a more permanent method of bonding a permanent magnet to a metal body. The method of this patent involves joining a ferrite ceramic magnetic body such as a nickel-zinc type ferrite ceramic to a metallic body such as titanium or steel by sputtering a thin layer of a chromium containing metal on a surface of the magnetic body, providing a layer of a silver containing brazing alloy between the chromium containing metal layer provided on the surface of the body and the metal body and forming a brazed bond by heating the assembly at a temperature at the range of 600.degree. C. to less than 1000.degree. C.
The major problem with the use of the method of this patent is that many magnetic materials are decomposed at the brazing temperatures employed. Further the use of such a use of a high temperature also frequently causes demagnetization of the magnets joined by this method.
Bronnes et al, U.S. Pat. No. 4,643,347 describes a method of magnetic permanent magnets to metal bodies by which a reliable permanent magnet to metallic body seal is achieved and in which the use of high brazing temperatures are avoided. According to this method of mounting permanent magnets are bonded to a metallic body such as a rotating or reciprocating shaft by providing a layer of titanium on a surface of the magnet, providing a barrier layer such as a nickel layer, copper alloy or a layer of a nickel-iron-cobolt alloy on the layer of titanium and providing a solderable layer such as copper, nickel or gold on the barrier layer. The metal body is then soldered to the solderable layer with the use of a low melting solder such as tin-lead solder.
However, the method of this patent has some drawbacks. It requires the use of an intermediate or barrier layer thus increasing the cost of forming the seal. In addition it has been found that the use of the method is limited to those magnets that do not suffer decomposition at the soldering temperature and which do not demagnetize at the soldering temperature.
The method of this patent is thus very well limited to mounting magnets of the aluminum nickel cobalt or the rare earth cobalt compound type.
It has been found for example that this method of this patent is not useful for mounting permanent magnets of the rare earth ferrite type, for example of the Nd-B-Fe variety. Thus it has been found that, under the conditions of the method of this patent, considerable demagnetization and decomposition of the magnet occurs.